This invention relates to actively stabilized lasers, and has particular application to lasers which avoid the use of dithering for frequency stabilization.
Lasers have a gain medium contained between a pair of mirrors. The lasing medium will sustain oscillations over only a narrow band of frequencies, producing considerable power at a central peak frequency, and dropping off in power production as the sides of the band are approached. The mirrors are spaced so that the optical length between them is an integral multiple of half wavelengths of the light of desired frequency. As this optical length drifts with age, temperature fluctuations, acoustics, vibrations, and the like, the frequency of the light produced by the laser drifts with it. If the laser is originally tuned to the peak frequency, this drift from the peak frequency results in reduced power. Further, if the laser is used to measure a distance or other parameter, this drift may cause measurement errors.
Passive stabilization--shock protection, constant temperature maintenance, and the like--can do only so much. Workers have therefore developed two forms of active stabilization.
For the first form of active stabilization, the lasing material is modified (as by applying a magnetic field to it, for Zeeman splitting), so as to have two separate modes of operation, separately detectable, with separate peak frequencies, and separate (but overlapping) frequency bands. The optical length is set so that the operating frequency is lower than the higher mode peak frequency, and higher than the lower mode peak frequency. As the operating frequency drifts, the ratio of the power produced in the high frequency mode to the power produced in the low frequency mode changes. This change in power ratio is detected, and the optical length is adjusted to restore the ratio to the desired valve. This restores the operating frequency to the desired value as well.
The second form of active stabilization is dithering. The frequency is deliberately changed--dithered--in a periodic manner, causing a concurrent fluctuation in the power output. If the operating frequency is above the peak frequency, then increasing the operating frequency will decrease the power output. Therefore, the power output fluctuation will be at the same frequency as, and a half cycle out of phase with, the frequency dither. Likewise, if the operating frequency is below the peak frequency, then increasing the operating frequency will increase the power output. Therefore, the power output fluctuation will be at the same frequency as, and in phase with, the frequency dither.
If the operating frequency is at the peak frequency, then power will be reduced whether the operating frequency is increased or decreased. Therefore, the power output fluctuation will be at twice the frequency of the frequency dither, and will give the appearance of inverted full wave rectification. When this second harmonic predominates over the first harmonic, operation is at (or near) peak frequency. The operating frequency is servo controlled to maximize the second harmonic, and minimize the first harmonic, of the dither frequency.